write Clang precompiled headers to a custom section

This doesn't fix the alignment issue yet.

See https://bugs.llvm.org/show_bug.cgi?id=35928 and
https://reviews.llvm.org/D42233.

Author: zhuowei

Diff for: lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -1672,6 +1672,10 @@ static SectionKind getWasmKindForNamedSection(StringRef Name, SectionKind K) {
   if (K.isText())
     return SectionKind::getText();
 
+  // Clang precompiled header data isn't needed at runtime; use custom section
+  if (Name == "__clangast")
+    return SectionKind::getMetadata();
+
   // Otherwise, ignore whatever section type the generic impl detected and use
   // a plain data section.
   return SectionKind::getData();

Diff for: lib/MC/WasmObjectWriter.cpp

@@ -1135,6 +1135,10 @@ static bool isInSymtab(const MCSymbolWasm &Sym) {
   if (Sym.isSection())
     return false;
 
+  // Clang's precompiled headers are in a separate custom section
+  if (Sym.getName() == "__clang_ast")
+    return false;
+
   return true;
 }